In 1987, Janet van Dyne joins Hank Pym on a mission to stop a Soviet nuclear missile launched toward the United States. Realizing there is no other way to disable it in time, Janet shrinks beyond the safe limit between molecules and enters the mysterious Quantum Realm. The missile is destroyed, but Janet is lost forever—or so Hank believes. Left devastated, Hank raises their young daughter Hope alone while mourning the wife he assumes can never return.
Years later, Scott Lang is serving the final days of a two-year house arrest sentence after violating the Sokovia Accords by assisting Captain America during the Avengers’ internal conflict. Though desperate to stay out of trouble, Scott has adjusted to confinement by entertaining his daughter Cassie, learning card tricks, and building a small security business with his friends Luis, Dave, and Kurt. With only three days left before his release, freedom finally seems within reach.
That hope changes when Scott experiences a vivid dream in which he relives one of Janet’s memories with young Hope. Believing the experience may mean something, he secretly contacts Hank Pym despite their estrangement. Scott quickly regrets it and destroys the hidden phone, but that night he is abruptly kidnapped. He awakens in the hidden mobile laboratory of Hope van Dyne, who remains angry that Scott used Hank’s Ant-Man suit without warning them, causing the authorities to target her and Hank as accomplices.
Despite their resentment, Hank realizes Scott’s vision coincided with the temporary activation of a quantum tunnel. He concludes that Scott and Janet have become quantumly entangled, proving Janet may still be alive. Hank and Hope reveal they have spent years building a portable laboratory and a stable tunnel that could send Hope into the Quantum Realm to retrieve her mother. However, they still need a critical component before the machine can function properly.
Hope arranges to buy the missing part from black-market businessman Sonny Burch, but Burch decides Pym’s technology is worth far more than the agreed price. He attempts to seize the lab and force them into business with him. Hope responds by unveiling her own advanced suit as the Wasp, equipped with wings, blasters, and superior maneuverability. She defeats Burch’s men in a dazzling fight, but before she can escape, a mysterious masked woman who can phase through walls attacks and steals Hank’s portable lab.
Scott joins the battle wearing a malfunctioning prototype Ant-Man suit that causes him to randomly grow and shrink. Though he and Hope recover the needed part, the masked woman escapes with the lab. With no choice, Hank reluctantly agrees to seek help from his estranged former partner, Dr. Bill Foster. At the University of California, Berkeley, Foster helps them understand how to track the lab, though old bitterness between him and Hank resurfaces.
While they pursue the lab, Scott realizes his old Ant-Man suit—still containing a needed regulator—is at Cassie’s school after she used it for show-and-tell. Scott and Hope sneak into the school to retrieve it while Scott uncontrollably shifts between child-size and giant-size due to the faulty suit. They barely escape before FBI agent Jimmy Woo, who is closely monitoring Scott’s house arrest, notices anything suspicious.
The trio finally locates the stolen lab in a remote forest, where they confront the masked thief. She reveals herself as Ava Starr, also known as Ghost. Ava explains that a failed experiment conducted by her father caused her molecules to become unstable, leaving her in constant pain and forcing her to phase unpredictably through matter. Foster has secretly been helping her survive and believes Janet’s quantum energy can cure her condition permanently.
Hank refuses to let Ava drain Janet’s energy, knowing it would likely kill her. After faking a heart attack to create a distraction, Scott, Hope, and Hank escape with the lab. They soon establish direct contact with Janet through Scott, whose mind briefly channels her consciousness. Janet gives them precise coordinates to find her and warns they have only a short window before the quantum pathway closes.
Complications worsen when Sonny Burch captures Luis and uses truth serum to learn where Scott can be found. Ghost overhears the conversation and learns of Janet’s rescue attempt. Scott rushes home to maintain the illusion that he never violated house arrest, but his absence allows authorities to arrest Hank and Hope while confiscating the lab. Ghost then murders the federal agent guarding it and steals the laboratory for herself.
Encouraged by Cassie, who reminds him that heroes help people no matter the risk, Scott chooses to act. He rescues Hank and Hope from custody, and the reunited team forms a final plan at San Francisco’s waterfront. They will let Ghost activate the lab and enlarge it, allowing Hank to enter the Quantum Realm while Scott and Hope defend the site from Ghost, Burch, and the FBI.
What follows is a chaotic chase through San Francisco involving shrinking cars, oversized PEZ dispensers, giant ants, and the portable lab rolling through city streets. Scott becomes Giant-Man to slow Ghost and Burch’s men, while Hope uses her speed and flight to outmaneuver pursuers. At Pier 39, Scott enlarges the lab, allowing Hank to travel into the Quantum Realm as the others hold off their enemies.
Inside the Quantum Realm, Hank finally finds Janet alive after thirty years. She has adapted to the strange dimension and developed new abilities from prolonged exposure to quantum energy. Husband and wife share an emotional reunion before returning safely to the real world. Hope tearfully embraces her mother, completing the family she thought was lost forever.
Ghost makes one final attempt to seize Janet’s energy, but Janet instead willingly channels some of her power into Ava, stabilizing her condition and easing her suffering. Burch and his men are arrested, and Woo reluctantly confirms that Scott completed his sentence without officially breaking the terms of house arrest. Hank restores the family beach house, and the reunited Pyms begin a hopeful new chapter.
In the mid-credits scene, Scott enters the Quantum Realm again to collect healing particles for Ghost while Hank, Janet, and Hope operate the controls. Just as they prepare to bring him back, all three vanish into dust as victims of Thanos’ Snap. Their disappearance leaves Scott trapped alone in the Quantum Realm with no way home.
In the post-credits scene, an oversized ant wearing Scott’s ankle monitor continues drumming in his bedroom while emergency news reports announce a global state of crisis, hinting at the catastrophic aftermath unfolding across Earth.
